Hollow Rods of Nanocrystalline NiGa2O4: Hydrothermal Synthesis

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Hollow Rods of Nanocrystalline NiGa2O4: Hydrothermal Synthesis, Formation Mechanism, and Application in Photocatalysis Hun Xue, Zhaohui Li,* Zhengxin Ding, Ling Wu, Xuxu Wang, and Xianzhi Fu* Research Institute of Photocatalysis, Fuzhou UniVersity, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou 350002, P. R. China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 12 4511–4516

ReceiVed May 16, 2008; ReVised Manuscript ReceiVed August 10, 2008

ABSTRACT: Hollow rods of NiGa2O4 were successfully prepared via a facile template-engaged reaction of nickel salt and commercially available rod-like Ga2O3. The samples were characterized by X-ray diffraction (XRD), UV-vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy dispersive X-ray spectrum (EDS). The results showed that the pH value, reaction time, and hydrothermal temperature have crucial effects on the formation of the phase and microstructure of the as-prepared NiGa2O4. The growth mechanism of the NiGa2O4 hollow rods was proposed. This facile and green method is proved to be a generic method for the one-step synthesis of hollow rods of other metal gallates, such as ZnGa2O4. The photocatalytic activity of RuO2-loaded NiGa2O4 was investigated by photocatalytic water splitting for hydrogen production.

1. Introduction To design and develop new materials with particular morphologies for more and more advanced applications has been one of the most important goals of materials scientists.1-4 Hollow structures, owing to their higher specific surface area, lower density, distinct optical properties and better permeation,4-6 show widespread potential applications in drug delivery,7 plasmonics,8 catalysis,9 photocatalysis,5,10,11 sensors,11,12 and various new application fields.4,5,13,14 Therefore, the preparation of materials with a hollow structure has attracted tremendous interest recently. Various methods, including hydrothermal,5,15,16 solvothermal,17 self-assembly,18 sonochemical,6,13,19,20 and template involved methods12,20-28 have been utilized in preparing inorganic materials with hollow structures. Among all the synthetic methods in the preparation of the hollow structures, the template-involved method has been proven to be the most effective. Hard templates, such as polymer latex particles,21 silica spheres,20 metal nanoparticles,23 and carbon spheres,12,22 together with soft templates, such as vesicles,25 emulsions,26,27 micelles,28 lipid24 and bacteria19 have been successfully used in the fabrication of hollow structures. However, the introduction of templates induces heterogeneous impurities, and the template removal process is usually energy-consuming and may cause the collapse of structures. Thus, developing a facile method in synthesis of hollow inorganic replicas with no additional core removal step is therefore of great importance and still a challenge for material scientists. Recently, a so-called template-engaged reaction has been demonstrated to successfully generate materials with a hollow nanostructure. In this method, the template is reacted with appropriate chemical reagents under carefully controlled conditions and is partially or completely converted to other materials without changing its original morphology. Xia et al. reported the preparation of CdSe nanotubes from cadmium salts and Se nanowires via this method.29 Xue et al. also reported that hollow microspheres of ZnO could be obtained via a spontaneous replacement reaction between Zn5(CO3)2(OH)6 microspheres and KOH solution.4 Although this approach can avoid the hetero* Author to whom all correspondence should be addressed. E-mail: [email protected] (Z.L.); [email protected] (X.F.); tel/fax: 86-59183738608.

geneous impurities introduced by the additional templates and it is energy-saving without the template removal process, the templates used so far are all synthesized via complicated chemical routes and the materials prepared are limited to binary metal compounds. Herein, we report the facile synthesis of spinel NiGa2O4 hollow rods via a template-engaged method from commercially available rod-like Ga2O3. The growth mechanism of the NiGa2O4 hollow rods was also proposed. To the best of our knowledge, this is the first report that the hollow structure of a ternary metal oxide has been prepared via this template-engaged method. Since NiGa2O4 belongs to a series of metal oxides composed of p-block metal ions with d10 configuration and many of these p-block metal oxides show photocatalytic activity for overall water splitting to produce H2, the photocatalytic performance for water splitting over the as-prepared NiGa2O4 was also studied. This is a generic, facile, and effective method in the preparation of the hollow rods of other metal gallates such as ZnGa2O4.

2. Experimental Procedures 2.1. Syntheses. The NiGa2O4 nanocrystalline samples were prepared by the hydrothermal method using reactants Ga2O3 as templates. All of the reactants and solvents were analytical-grade and were used without any further purification. Commercially available rod-like Ga2O3 with diameters of 1-5 µm and lengths ranging from 5 to 20 µm were purchased from Rongruida powder material factory, Zibo, Shangdong. In a typical procedure, Ga2O3 powder (2 mmol) were added to 75 mL of aqueous solution containing 2 mmol of Ni(NO3)2 · 6H2O under stirring. The pH of the resulting mixture was adjusted to 10 with sodium hydrate solution (4 mol · L-1) under vigorous stirring. The resulting suspension was transferred into a 100 mL Teflon-lined stainless steel autoclave and sealed tightly. Then, the autoclave was kept at 180 °C for 36-120 h. After cooling of the sample to room temperature, the light blue precipitates were collected, washed with distilled water and absolute ethanol several times, and dried in air at 80 °C. The preparation of 1 wt% RuO2-loaded NiGa2O4 was performed by an impregnation method using Ru3(CO)12 (Aldrich, 99%) as starting materials. The prepared NiGa2O4 was impregnated with Ru3(CO)12 in THF, dried at 80 °C, followed by oxidation in air at 500 °C for 5 h to convert the loaded Ru complex into dispersed RuO2 particles. 2.2. Characterizations. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer with CuKR

10.1021/cg8005162 CCC: $40.75  2008 American Chemical Society Published on Web 10/10/2008

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Figure 1. Images of NiGa2O4 hollow rods obtained with pH ) 10 at 180 °C for 36 h. (a) XRD pattern; (b, c) SEM images; (d) TEM image; (inset) SAED pattern recorded on the shell of hollow rod; (e) HRTEM image; (f) the corresponding EDX spectrum taken on hollow rod. radiation. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. Data were recorded at a scanning rate of 0.02° s-1 in 2θ ranging from 10° to 80°. It was used to identify the phase present. UV-visible absorption spectra of the powders were obtained for the dry-pressed disk samples using a UV-visible spectrophotometer (Cary 500 Scan Spectrophotometers, Varian, USA). BaSO4 was used as a reflectance standard in the UV-visible diffuse reflectance experiments. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were measured by a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. The powder particles were supported on a carbon film coated on a 3 mm diameter fine-mesh copper grid. A suspension in ethanol was sonicated and a drop was dripped on the support film. Morphology of the sample was characterized by field emission scanning electron microscopy (SEM) (JSM-6700F). 2.3. Photocatalytic Activity Measurements. The photocatalytic reactions were performed using a closed system with an innerirradiation-type quartz reactor. A 125 W high-pressure mercury lamp (Shanghai Yaming Lamp Works) was the light source. The lamp was plunged in a quartz immersion well with cooling water. The temperature of the reaction cell was controlled at about 30 °C by cooling water. Briefly, 1 wt% RuO2-loaded NiGa2O4 powder (200 mg) was placed in distilled water (200 mL) in the reactor. The closed gas circulating line was vacuumized by a pump, and then was filled with nitrogen to common press. This was repeated several times till oxygen was not detected. The suspension was mixed with the magnetic stirrer for 30 min, and then the UV lamp was turned on. With an increase in

Figure 2. XRD patterns of the samples prepared with pH ) 10 for 36 h at 160 °C; 180 and 200 °C. (V) NiGa2O4, (f) Ga2O3, (b) 3Ni(OH)2 · 2H2O. irradiation time, gases were evolved, and the gas pressure was increased. The gases were circulated with a pump during the reaction. The gases were analyzed in situ with an online gas chromatograph (GC112A, Shanghai precision and scientific instrument Co., LTD, nitrogen carrier) equipped with a carbon molecular sieve (TDX-01) column and a thermal conductivity detector, which was connected to the closed gas circulating line. The production of H2 from aqueous CH3OH solution (10 vol%) dispersed with 1 wt% RuO2-loaded NiGa2O4 powder was also observed.

Hollow Rods of Nanocrystalline NiGa2O4

Figure 3. SEM image of the sample (with pH ) 10 at 200 °C for 36 h).

Figure 4. XRD patterns of the samples prepared at 180 °C for 48 h with pH ) 8 and pH ) 10. (V) NiGa2O4, (f) Ga2O3, (b) 3Ni(OH)2 · 2H2O.

3. Results and Discussion 3.1. Preparation and Characterizations of NiGa2O4 Hollow Rods. Hydrothermal treatment of the reaction mixture at 180 °C in a solution with pH ) 10 for 36 h led to the formation of NiGa2O4. The XRD pattern of the as-prepared sample shows peaks at 2θ values of 18.6°, 30.6°, 36.0°, 37.7°, 43.8°, 54.4°, 58.0°, 63.7°, 72.3°, 75.4°, and 76.4°, which correspond to (111), (220), (311), (222), (400), (422), (511), (440), (620), (533), and (622) crystal planes of cubic NiGa2O4 with spinel structure (Figure 1a). All the peaks matched well with the characteristic reflections of NiGa2O4 (JCPDS card 780546). No peak attributable to other phases is observed and indicates the formation of the pure phase of NiGa2O4. The SEM image shows that the as-prepared sample consists almost entirely of rods with diameters of 1-5 µm and lengths ranging from 3 to 8 µm (Figure 1b). An enlarged SEM image from a broken rod shows that the rod is hollow and the wall of the hollow rod is composed of numerous tightly aggregated polyhedrons as displayed in Figure 1c. The relative pale center together with the dark edge observed on the TEM image of a single rod further confirms the hollow structure of the as-prepared NiGa2O4 rods (Figure 1d). The HRTEM image shows clear lattice fringes. The fringes of d ) 0.29 nm matches that of the (220) plane of the cubic NiGa2O4 (Figure 1e). The selected area electron diffraction (SAED) pattern taken from a single polyhedron could be indexed as a cubic NiGa2O4 single crystal (inset in Figure 1d). The energy-dispersive X-ray (EDX) analysis of the as-prepared hollow rod shows that the atomic percentage of Ni and Ga are

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21.65% and 46.42% respectively, indicating that the atomic ratio of Ni and Ga is about 1:2 (Figure 1f), which is in agreement with the expected stoichiometry of NiGa2O4. 3.2. Influence Factors. Temperature. Temperature is found to play an important role in the crystallization and shape control of NiGa2O4. For the samples prepared with pH ) 10 for 36 h, when the reaction temperature was 160 °C, the products were R-3Ni(OH)2 · 2H2O (JCPDS file 22444) and Ga2O3, together with small amounts of NiGa2O4 as shown in Figure 2. Pure NiGa2O4 could not be obtained at a reaction temperature lower than 180 °C, probably due to the difficulty in hydrolyzing Ga2O3 to form Ga(OH)4- species in solution at such a low reaction temperature.30 Although pure NiGa2O4 can be obtained at a reaction temperature higher than 180 °C (Figure 2), the SEM image of the sample prepared at 200 °C shows that the hollow rods collapsed and conglomerated (Figure 3). pH. Controlled experiments were carried out to investigate the influence of pH on the morphology and composition of the final products. For the samples prepared at 180 °C for 48 h, the resulting products were R-3Ni(OH)2 · 2H2O and Ga2O3, together with small amounts of NiGa2O4 in a solution with a low pH value (pH ) 8) (Figure 4). The TEM and SEM images of the resulting product as shown in Figure 5a-e show that many hexagonal sheets with a thickness of about 40-50 nm are lying on the unreacted Ga2O3 rods. The nanosheets are confirmed to be R-3Ni(OH)2 · 2H2O since the HRTEM image of the nanosheet shows clear lattice fringes of d ) 0.26 nm, which matches that of the (110) crystallographic planes of R-3Ni(OH)2 · 2H2O (Figure 5f). The inability of obtaining NiGa2O4 in a solution with pH ) 8 is not surprising since Ga2O3 is difficult to hydrolyze to form the Ga(OH)4- species in a solution with a low pH value.30 Pure NiGa2O4 hollow rods could only be obtained in a solution with a pH value larger than 10. Time. Time-dependent experiments were carried out to clarify the growth process of the as-prepared NiGa2O4 hollow rods. For reaction carried out at 180 °C in a solution with pH ) 10, a reaction time of 24 h gave mixed products of R-3Ni(OH)2 · 2H2O, Ga2O3 and spinel NiGa2O4 (Figure 6). Although pure NiGa2O4 could be obtained in such a system for a reaction time longer than 36 h (Figure 6), the reaction time could influence the morphology of the final products. A longer reaction time can lead to the collapse of the hollow rods. The TEM and SEM images of the sample obtained from a 72 h reaction display the coexistence of the broken hollow rods and octahedra (Figure 7a-d). It indicates that the broken rods are made of the octahedra. So, we presume that the polyhedrons that the NiGa2O4 hollow rods consist of are also octahedral. The selected area electron diffraction (SAED) pattern taken from a single octahedron confirms that the octahedron is a perfect single crystal (inset in Figure 7d). Only conglomerated octahedra can be observed on the SEM image of the product obtained after a 120 h reaction (Figure 8). 3.3. Formation Mechanism. Based on the above observations, a possible mechanism for the formation of NiGa2O4 hollow rods via the rod-like Ga2O3 is outlined in Figure 9. The possible chemical reactions involved in the synthesis of NiGa2O4 can be formulated as follows:

3Ni(OH)2+ 2H2O f R-3Ni(OH)2·2H2O

(1)

Ga2O3+ 3H2O f 2Ga(OH)3

(2)

Ga(OH)3+ OH- f Ga(OH)4

(3)

R-3Ni(OH)2·2H2O + 6Ga(OH)4 f 3NiGa2O4+ 14H2O + 6OH- (4) In basic condition, the nuclei of R-3Ni(OH)2 · 2H2O form (eq 1) and deposit on the rods of Ga2O3 (step 1). Ga2O3 is

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Figure 5. Images of NiGa2O4 obtained with pH ) 8 at 180 °C for 48 h (a-c) SEM images; (d, e) TEM images; (f) HRTEM image.

Figure 6. XRD patterns of the samples prepared with pH ) 10 at 180 °C for 24 h; 36 h; 72 and 120 h. (V) NiGa2O4, (f) Ga2O3, (b) 3Ni(OH)2 · 2H2O.

hydrolyzed to give Ga(OH)4- species (eqs 2 and 3). The asformed R-3Ni(OH)2 · 2H2O nuclei can serve as the nucleation centers for the subsequent reaction between R-3Ni(OH)2 · 2H2O and Ga(OH)4- to give NiGa2O4 nanoparticles covering the Ga2O3 rods (eq 4). This reaction continues and NiGa2O4 layer is formed on the original rods until the reaction is quenched or

all Ga2O3 are consumed. The replica of the original Ga2O3 rods can be obtained and the hollow rods of NiGa2O4 are formed (step 2). The incapable of obtaining NiGa2O4 at lower pH value (pH ) 8) or a lower temperature (160 °C) may be ascribed to the difficulty in hydrolyzing Ga2O3 to give Ga(OH)4- species under such conditions.30 Similar hollow rods of other metal gallates such as ZnGa2O4 can also be successfully prepared following a similar method. 3.4. Photocatalytic Activity. Since NiGa2O4 belongs to a series of metal oxides composed of p-block metal ions with d10 configuration, it is anticipated that it would show photocatalytic activity for the water splitting to produce H2.31-34 Therefore, the photocatalytic H2 evolution over 1 wt% RuO2loaded hollow rods of NiGa2O4 was investigated, and the results were shown in Figure 10a. It shows that the average evolution rate of H2 is about 1.25 × 104 nmol · g-1 · h-1 when pure water is used as the medium. The amount of evolved H2 increased with the irradiation time, while no gases evolved upon turning off the light. It confirmed that the gas evolution is intrinsically a photocatalytic process. The H2 evolution rate could be improved to 2.20 × 105 nmol · g-1 · h-1 when a mixture of water

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Figure 7. Images of NiGa2O4 obtained with pH ) 10 at 180 °C for 72 h (a, b) SEM images; (c, d) TEM images; (inset) SAED pattern recorded on the octahedron.

Figure 8. SEM image of the sample (with pH ) 10 at 180 °C for 120 h).

Figure 9. Mechanism for the formation of NiGa2O4 hollow rods.

and methanol was used as the medium (Figure 10b). The rate of H2 evolution increases greatly in the presence of methanol as an electron donor.

4. Conclusions Hollow rods of NiGa2O4 have been successfully prepared via a facile template-engaged reaction from nickel salt and commercially available rod-like Ga2O3. The simplicity of the hydrothermal process and the commercial availability of the raw

Figure 10. (a) Photocatalytic H2 evolution from pure water (200 mL) dispersed with 1 wt% RuO2-loaded NiGa2O4 (0.2 g); (b) photocatalytic H2 evolution from the mixture solution of water (180 mL) and methanol (20 mL) over 1 wt% RuO2-loaded NiGa2O4 (0.2 g) under UV light irradiation; light source: 125 W high-pressure Hg lamp.

materials are the advantages favoring the scaling-up of hollow rods of NiGa2O4. This method is proved to be a generic method for the one-step synthesis of hollow rods of other metal gallates, such as ZnGa2O4. RuO2-loaded NiGa2O4 shows photocatalytic activity for water splitting to produce H2.

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Acknowledgment. The work was supported by National Natural Science Foundation of China (20537010, 20677009), National Basic Research Program of China (973 Program: 2007CB613306, 2007CB616907), grant from Fujian Province (E0710009). Z.L. thanks program for New Century Excellent Talents in University (NCET-05-0572), State Education Ministry of P. R. China. Supporting Information Available: Preparation of ZnGa2O4 hollow rods, XRD patterns, and SEM images of commercial Ga2O3 rods and ZnGa2O4 hollow rods. These materials are available free of charge via the Internet at http://pubs.acs.org.

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